The C. glabrata ERG11 gene, encoding the cytochrome P-450-dependent `-14 demethylase, was deleted in order to assess the effect on sterol synthesis, aerobic viability and azole susceptibility. A mutant in which both ERG3 and ERG11 was deleted was aerobically viable and accumulate 14methylfecosterol. A mutant in which only ERG11 was deleted was not aerobically viable but gave rise to an aerobically viable revertant which accumulated both lanosterol and obtusifoliol. This ERG11 mutant had an intact ERG3 gene and increased ERG3 message on Northern analysis. This contrasts with S. cereviseae, in which aerobically viable ERG11 revertants have no functional ERG3 gene. The ERG11 revertant and the ERG3, ERG11 mutant were resistant to azoles, slightly more resistant to amphotericin B and more readily killed by human neutrophils or hydrogen peroxide. We have enrolled four patients in our study of oropharyngeal candidiasis in AIDS patients. One of the four had increased azole resistance in both his C. albicans and C. glabrata isolates at time of relapse. The mechanism by which resistance occurred is under study.